The present invention relates generally to an image transfer device including image transfer members for transferring an image from an image receiving member onto a recording medium and, more particularly, to an image transfer device for transferring an image onto a recording medium with the aid of an electric field applied by an endless belt having a specific value of electrical resistance.
There exists a conventionally known electrophotographic image forming apparatus provided with an image transfer device which includes image transfer members and an endless belt, in which the image transfer members are located to face an image receiving member via the endless belt, the image receiving member carrying an image visualized by a developer. With a specific voltage applied, the image transfer members create an image transfer electric field. A recording medium onto which the image is to be transferred is fed onto a primary side of the endless belt facing the image receiving member and transported between the image receiving member and the image transfer members as the endless belt turns with the recording medium adhering to the primary side of the endless belt. The visual image on the image receiving member is transferred onto the recording medium by the electric field applied by the endless belt through the recording medium.
In the image transfer device thus structured, the intensity of the electric field applied at an image transfer position varies with such properties of the recording medium as the thickness and water content thereof. With this arrangement, the voltage applied to the image transfer members is controllably varied according to the properties of the recording medium in some conventional image transfer devices as disclosed on Japanese Laid-Open Patent Application Nos. H03-225384 and H05-11625.
To control the applied voltage according to the properties of the recording medium in this manner, however, there is the need for the provision of a power supply capable of varying the supply voltage over a wide range from a low voltage to a high voltage. One conventional approach to varying the supply voltage of a single power supply over a wide range from a low voltage to a high voltage in this fashion is to ground the image transfer members via a variable resistor. This approach, however, has such problems as an increase in size and manufacturing cost of the image transfer device due to the need for a wide variable range of the variable resistor, as well as a risk of excessive heat generation as a result of a change in the supply voltage.